It has long been known that silica-based optical fibers are susceptible to stress-assisted corrosion, also known as static fatigue, when the fiber is under stress in the presence of moisture. One approach to this problem has been to manufacture fiber having an outer layer which is under relatively high compressive stress. A stronger fiber is believed to result from such built-in stress. A method for making such fibers is disclosed, for example, in U.S. Pat. No. 4,243,298, issued on Jan. 6, 1981, to C. K. Kao, et al.
Optical fibers having high-compression outer layers may have certain disadvantages. For example, fiber joints are conventionally made in an operation which includes the cleaving of the fiber ends that are to be joined. A predetermined tension is applied to each fiber by the cleaving machine in order to fracture the fiber at a desirable cleave angle, i.e., an angle which is preferably 0.0.degree., but which is more typically about 0.25.degree.. It has been observed that in order to produce a desirable cleave angle in a fiber having a high-compression outer layer, the cleaving tension must be increased. This requirement is undesirable, because the extra step that is entailed is inconvenient for workmen making field installations, increases the cost of such installations, and increases the likelihood of error.
Practitioners have hitherto failed to provide a silica-based optical fiber having a modified outer glass layer which offers improved resistance to static fatigue, but which is not in substantially higher compression than other regions of the fiber.